1. Field of the Invention
The invention relates to a matrix print head with individual dot-print elements, with electromagnetic circuits coordinated to the individual dot-print elements, where the dot-print elements, in each case, are attached to spring armatures or anchors which, in each case, are disposed opposite to a coil core of an electromagnetic coil and where the armature, the core of the coil, a back plate, a permanent magnet, and a yoke plate form a main-series magnetic circuit, where the armature in the rest position rests on the coil core.
2. Brief Description of the Background of the Invention Including Prior Art
Such matrix print heads require a high writing capacity and, at the same time, a long lifetime and durability. The requirements necessary for this purpose aim at a high degree of effectiveness of the magnetic flux circuit and at achieving in continuous operation a low heating of the matrix print head through low power dissipation.
A matrix print head is known from the German Patent DE-PS 3,110,798. However, the construction according to this patent is associated with the disadvantage that the magnetic flux of a permanent magnet is lower in a high-temperature state as compared to room temperature. The known teaching furthermore has as a starting point to consider the stable operation of the print head at the high temperature as a most substantial viewpoint. Correspondingly, the object of the conventional teaching is to improve a print head of the kind indicated there such that the print head can operate even at high temperatures in a continuous manner, at a high speed, and by providing a good printing quality.
In contrast, the following points have to be considered also. A magnetic circuit of the kind of the reference is associated with substantial dimensional variations and with deviations based on the manufacturing and material tolerances of the individual parts as well as based on the assembly procedures. For example, unevenesses of the back plate and of the yoke ring can occur. During assembly, different rest positions of the armature occur in the magnetic circuit from armature to armature, which are based on so-called joining tolerances. In addition, one has to expect magnetic tolerances of the permanent magnets, i.e. there is a changing remanence and a changing coercive force. The same holds for the soft-magnetic material, where the desired homogeneity cannot be achieved during their production process. All imponderables taken together result finally in an inadequate printed writing based on different magnetic circuit forces in conjunction with the dynamic part of the armature plate of the spring armature. Forced or compulsory overdimensioning of the parts and of the forces results in a negative influence on the functioning. Other known matrix print head constructions have already attempted to resolve this problem by magnetizing and weakening of the magnet to a certain operating point of the magnet. However, for this purpose, expensive and complicated apparatus is required for tuning of the operating point.